The present invention relates to a method of inspecting a pattern and an apparatus thereof, in which a defect or a candidate for a defect is inspected by obtaining an image indicating physical properties of objects such as a semiconductor wafer, a TFT, and a photo mask with the use of an electron beam or a light beam, and by then comparing the image with an image obtained differently. More particularly, the present invention relates to a method of inspecting a pattern and an apparatus thereof, in which a microscopic defect or a candidate for a microscopic defect is inspected.
Concerning an inspection for recognizing a defect of a pattern, known as a prior art technology 1, as disclosed in Japanese Laid-Open Patent Publication No. 57-196377, is as follows. First, a pattern of an object to be inspected, such as a semiconductor including a pattern with a repetition, is detected, and the pattern detected is stored. Then, position alignment is performed in a pixel or picture element unit between the detected pattern and a pattern stored in advance. Finally, a defect is recognized by extracting a mismatch between the two patterns over which the position alignment has been made. Also, known as a prior art technology 2, as disclosed in Japanese Laid-Open Patent Publication No. 3-177040, is a technique which makes it possible to improve, of a problem of a mismatch between the both images in a normal part, a portion of the problem that is attributed to a missing of a position at which the both images are to be detected. Namely, described in the prior art technology 2 is the following technique. First, a pattern of an object is detected as an image signal. Then, position alignment is performed in a pixel unit between the image signal of the detected pattern and an image signal of the pattern stored in advance or an image signal of the pattern detected differently. Still then, with one pixel or less of accuracy, additional position alignment is further made between the image signals over which the position alignment has been made in the pixel unit. Finally, a defect of the pattern is recognized by extracting and comparing errors of the image signals of the two patterns over which the additional position alignment has been made with one pixel or less of accuracy.
Between the two images to be compared, even in the normal part, there exists differences such as an infinitesimal difference in pattern configuration, a difference in a gradation or tone value, a distortion of the pattern, and a shift of the position, which are attributed to the object to be inspected and an image detecting system. Namely, the mismatch in the normal part is classified into a mismatch due to the object to be inspected and a mismatch due to the image detecting system. The mismatch due to the object to be inspected is caused by a subtle difference in repetition patterns produced through a wafer manufacturing process such as an exposure, a developing and an etching. This, on the detected images, appears as the infinitesimal difference in pattern configuration and the difference in the gradation value. The mismatch due to the image detecting system is caused by variations in illuminating light quantity, an oscillation of a stage, a variety of electrical noises, and a missing of the position at which the both images are to be detected. These, on the detected images, appear as a difference in a gradation value of partial images, the distortion of the pattern, and the position shift of the images.
In the above-mentioned prior art technology 1, there existed the following problem. Since the above-described factors cause the mismatch to occur even in the normal part, if every mismatched portion is judged to be a defect, it turns out that there occurs a lot of false information. If, in order to prevent this, a criterion for the defect judgement is lowered, it becomes impossible to detect a microscopic defect.
Also, in the above-mentioned prior art technology 2, of the infinitesimal difference in the pattern configuration, the difference in the gradation value, the distortion of the pattern, and the position shift, which are attributed to the object to be inspected and the image detecting system, there existed an effect of reducing an influence of the position shift between the images. The other differences, however, were not taken into consideration.
It is an object of the present invention to provide, in order to solve the above-mentioned problems accompanied by the prior arts, a method of inspecting a pattern and an apparatus thereof, which make it possible to further reduce the number of false information caused by a mismatch due to an object to be inspected and an image detecting system, and to detect a more microscopic defect or a candidate for a more microscopic defect.
It is another object of the present invention to provide, when a pattern formed on an object to be inspected is inspected by means of an electron microscope, a method of inspecting the pattern and an apparatus thereof, which make it possible to further reduce the number of false information caused by a mismatch due to the object to be inspected and an image detecting system, and to detect a more microscopic defect or a candidate for a more microscopic defect.
It is still another object of the present invention to provide a method of inspecting a pattern and an apparatus thereof, which make it possible to obtain an image signal having a stable gradation value (a value of light and shade) from an inspection through an electron microscope of a pattern formed on an object to be inspected, and to stably detect a more microscopic defect or a candidate for a more microscopic defect.
In order to attain the above-described object, the present invention is characterized by the configuration described below, when a defect or a candidate for a defect of an object to be inspected is detected by comparing a first two-dimensional image, on each pixel of which a gradation value is a sampling value of a physical quantity at each point in one area to be inspected on the object to be inspected, with a second two-dimensional image, on each pixel of which a gradation value is a sampling value at each point in the other area which is supposed to have the same physical quantity as the above-mentioned physical quantity.
Namely, the present invention is characterized by comprising the steps of performing position alignment between the first two-dimensional image and the second two-dimensional image in a pixel unit, calculating one pixel or less of position shift quantity of corresponding portions between the first two-dimensional image f1(x, y) and the second two-dimensional image g1(x, y) over which the position alignment has been made in the pixel unit, determining for each pixel sub(x, y), i.e. a distance (difference) between a partial image (an image in a smaller area) in the first two-dimensional image and a partial image (an image in a smaller area) in the second two-dimensional image which have the one pixel or less of position shift quantity, determining for each pixel an allowable range (threshold values thH(x, y) and thL(x, y) at an upper limit and a lower limit, respectively) of the distance (difference) from the one pixel or less of position shift quantity determined and gradation values of the first two-dimensional image and the second two-dimensional image, checking whether or not the above-described distance (difference) between the partial images falls within the above-described allowable range for each pixel, and judging a pixel, on which the distance between the partial images is within the allowable range, to be an non-defective candidate and a pixel, on which the distance between the partial images is beyond the allowable range, to be a defective candidate.
Also, the present invention is characterized by comprising the steps of performing position alignment between the first two-dimensional image and the second two-dimensional image in a pixel unit, calculating one pixel or less of position shift quantity of corresponding portions between the first two-dimensional image and the second two-dimensional image over which the position alignment has been made in the above-mentioned pixel unit, determining a distance (difference) between the first two-dimensional image and the second two-dimensional image in each of a plurality of states in which the position relationship between the first two-dimensional image and the second two-dimensional image is shifted in a variety of directions by predetermined quantities determined by xcex1, xcex2 from a state in which the above-stated one pixel or less of position shift quantity has been amended, comparing with each other a distance (difference) between partial images in the both two-dimensional images determined for each of the above-mentioned position relationships in the plurality of states and determining for each pixel a maximum and a minimum of the distances between the partial images, determining for each pixel an allowable value (threshold values at an upper limit and a lower limit) of the distances between the partial images from a gradation value of the first two-dimensional image and that of the second two-dimensional image over which the position alignment has been made in the pixel unit, comparing a value obtained by adding the allowable value to the maximum with a value obtained by subtracting the allowable value from the minimum, and judging a pixel, on which the signs of them are opposite (the polarity is different), to be an non-defective candidate and a pixel, on which the signs of them are same (the polarity is same), to be a defective candidate.
Besides, the present invention provides a method and an apparatus for inspecting a defect or a candidate for a defect of an object to be inspected, by comparing a first two-dimensional image, on each pixel of which detected as a gradation value is a sampling value of a physical quantity at each point on the object to be inspected, with a second two-dimensional image on each pixel of which represented as a gradation value is a sampling value. Incidentally, in this invention, the second two-dimensional image is employed as an object compared with the first two-dimensional image. In addition, the present invention is characterized in that an allowable range is calculated in correspondence with a position shift quantity between an image in a predetermined area in the first two-dimensional image and an image in a predetermined area in the second two-dimensional image, a distance or a difference between the above-mentioned first two-dimensional image and the above-mentioned second two-dimensional image is determined using the gradation values, and a pixel is judged to be the defect or the candidate for the defect depending on whether or not the distance or the difference determined using the gradation values is within the calculated allowable range described above.
In addition, in the above-described method of inspecting a pattern and the apparatus thereof, the present invention is characterized by calculating a local change rate of a gradation value, using any one of the following formulas 14-1, 14-2, and 14-3, or enlarging the formulas 14-1, 14-2, and 14-3 up to n*n pixel in proximity to a pixel to which attention is being given. Assuming that f (x, y) is a gradation value of the pixel to which attention is being paid, and d x(x, y) and d y(x, y) are local change rates of the gradation value in a x- direction and a y-direction, respectively, the formulas to be used are as follows.
dx(x,y)=f(x+1,y)xe2x88x92f(x,y) 
dy(x,y)=f(x,y+1)xe2x88x92f(x,y)xe2x80x83xe2x80x8314-1 
dx(x,y)=[{f(x+1,y)+f(x+1,y+1)}xe2x88x92{f(x,y)+f(x,y+1)}]/2 
dy(x,y)=[{f(x,y+1)+f(x+1,y+1)}xe2x88x92{f(x,y)+f(x+1,y)}]/2xe2x80x83xe2x80x8314-2 
dx(x,y)=dy(x,y)=max{f(x,y), f(x+1,y), f(x,y+1), f(x+1,y+1)}xe2x88x92min{f(x,y), f(x+1,y), f(x,y+1), f(x+1,y+1)}xe2x80x83xe2x80x8314-3 
Moreover, in the above-described method of inspecting a pattern and the apparatus thereof, the present invention provides a method of calculating a variation allowable range of the gradation value, which is to be determined as a function of the local change rate of the gradation value, with the use of either of the following formulas 15-1, 15-2.
15-1 an upper limit in a variation allowable range of a gradation value at a coordinate (x,y)=|dx(x,y)*xcex1|+|dy(x,y)*xcex2|
a lower limit in a variation allowable range of a gradation value at a coordinate (x,y)=xe2x88x92|dx(x,y)*xcex1|xe2x88x92|dy(x,y)*xcex2|
15-2 an upper limit in a variation allowable range of a gradation value at a coordinate (x,y)={square root over ( )}{(dx(x,y)*xcex1)2+(dy(x,y)*xcex2)2}
a lower limit in a variation allowable range of a gradation value at a coordinate (x,y)=xe2x88x92{square root over ( )}{(dx(x,y)*xcex1)2+(dy(x,y)*xcex2)2}
,assuming that dx(x,y) and dy(x,y) are the local change rates of the gradation value in a x- direction and a y-direction, respectively, which are calculated from gradation values of a plurality of pixels in proximity to the pixel to which attention is being paid, and xcex1, xcex2 are real numbers greater than zero.
Furthermore, in the above-described method of inspecting a pattern and the apparatus thereof, the present invention provides a method of treating, in such a manner as to be described in any one of the following methods 16-1, 16-2, and 16-3, the variation allowable range of the gradation value which is to be determined as a function of a representative gradation value.
16-1 Employed as the representative gradation value is a maximum of the gradation value within a range of xcfx81*xcfx81 pixel in proximity to a pixel (x, y) to which attention is being given, and the variation allowable range is determined as a function of the representative gradation value.
16-2 Employed as the representative gradation value is an average value of the gradation value within a range of xcfx81*xcfx81 pixel in proximity to a pixel (x, y) to which attention is being given, and the variation allowable range is determined as a function of the representative gradation value.
16-3 A look-up table for the representative gradation value is prepared beforehand, and, in accordance therewith, the variation allowable range is determined.
Further, in the above-described method of inspecting a pattern and the apparatus thereof, the present invention is characterized in that a method of superposing values obtained by a plurality of methods out of the above-described methods (1), (2), and (3) is as follows. Summing up a plurality of values obtained by a plurality of methods out of the methods (1), (2), and (3), taking the square root of summation of squares of a plurality of values out of the methods (1), (2), and (3), or taking the square root of summation of a square of summation of a plurality of values out of the methods (1), (2), and (3), and a square of summation of the other plurality of values.
Still further, in the above-described method of inspecting a pattern and the apparatus thereof, the present invention is characterized by performing part of the whole process or the whole process with a plurality of pixels as one pixel.
Even further, in the above-described method of inspecting a pattern and the apparatus thereof, the present invention is characterized by embodying content described with a plurality of pixels as one pixel.